In English, a lexical distinction is drawn between the indefinite determiner “a” and the numeral “one”. English-speaking children also interpret the two terms differently, with an exact, upper bounded interpretation of the numeral “one”, but no upper bounded interpretation of the indefinite determiner “a”. Unlike English, however, German does not draw a distinction between the indefinite determiner and the numeral one but instead uses the same term “ein/e” to express both functions. To find out whether this cross-linguistic difference affects children’s upper bounded interpretation of “ein/e”, we tested German-speaking children and adults in a truth-value-judgment task and compared their performance to English-speaking children. Our results revealed that German-speaking children differed from both English children and German adults. Whereas the majority of German adults interpreted “ein/e” in an upper bounded way (i.e. as exactly one, not two), the majority of German-speaking children favored a non-upper bounded interpretation (thus accepting two as a valid response to “ein/e”). German-speaking children’s proportion of upper bounded responses to “ein/e” was also significantly lower than English children’s upper bounded responses to “one”. However, German children’s rate of upper bounded responses increased once a number-biasing context was provided. These findings suggest that German-speaking children can interpret “ein/e” in an upper bounded way but that they need additional cues in order to do so. When no such cues are present, German-speaking children differ from both German-speaking adults and from their English-speaking peers, demonstrating that cross-linguistic differences can affect the way speakers interpret numbers.